FIG. 1 is a schematic view of the human heart. The human heart 10 is a muscular dual pump that beats continuously throughout life sending blood to the lungs and the rest of the body. The interior of the heart consists of four distinct chambers. The septum 12, a thick central muscular wall, divides the cavity into right and left halves. On the right side, the upper half is known as the right atrium 14. Deoxygenated blood from the rest of the body arrives in the right atrium via the vena cava 16, the blood is pumped across a one-way valve known as the tricuspid valve 18 into the lower portion known as the right ventricle 20. From there the blood circulates to the lungs through the pulmonary valve 22 via the pulmonary artery 24 where it is oxygenated by circulation through the alveoli of the lungs (not shown). The blood returns via the pulmonary veins to the left atrium 26 and flows through a second valve, the mitral valve 28 into the left ventricle 30 where it is pumped via the aorta 32 to the rest of the body.
Much of the heart consists of a special type of muscle called myocardium. The myocardium requires a constant supply of oxygen and nutrients to allow it to contract and pump blood throughout the vasculature. The inner surfaces of the chambers of the heart are lined with a smooth membrane, the endocardium, and the entire heart is enclosed in a tough, membranous bag known as the pericardial sac.
The pumping action of the heart has three main phases for each heart beat. Diastole is the resting phase during which the heart fills with blood: while deoxygenated blood is entering the right atrium oxygenated blood is returned from the lungs to the left atrium. During the atrial systole, the two atria contract simultaneously, squeezing the blood into the lower ventricles. Finally, during ventricular systole the ventricles contract to pump the deoxygenated blood into the pulmonary arteries and the oxygenated blood into the main aorta. When the heart is empty, diastole begins again. The electrical impulses which stimulate the heart to contract in this manner emanate from the heart's own pacemaker, the sinoatrial node. The heart rate is under the external control of the body's autonomic nervous system.
FIG. 2 is a schematic view of the coronary arteries on the outer surface of the human heart Though the heart supplies blood to all other parts of the body, the heart itself has relatively little communication with the oxygenated blood supply. Thus, the two coronary arteries, the left coronary artery 40 and the right coronary artery 42 which arise from the aorta and encircle the heart muscle on either side "like a crown" to supply the heart itself with blood.
Heart disorders are a common cause of death in developed countries. They also impair the quality of life of millions of people restricting activity by causing pain, breathlessness, fatigue, fainting spells and anxiety. The major cause of heart disease in developed countries is impaired blood supply. The coronary arteries, which supply blood to the heart, become narrowed due to atherosclerosis and part of the heart muscle are deprive of oxygen an other nutrients. The resulting ischemia or blockage can lead to angina pectoris, a pain in the chest, arms or jaw due to a lack of oxygen to the heart, or infarction, death of an area of the myocardium caused by the ischemia.
Techniques to supplement the flow of oxygenated blood directly from the left ventricle into the myocardial tissue have included needle acupuncture to create transmural channels (see below) and implantation of T-shaped tubes into the myocardium. Efforts to graft the omentum, parietal pericardium, or mediastinal fat to the surface of the heart had limited success. Others attempted to restore arterial flow by implanting the left internal mammary artery into the myocardium.
Modernly, coronary artery blockage can be relieved in a number of ways. Drug therapy, including nitrates, beta-blockers, and peripheral vasodilator drugs (to dilate the arteries) or thrombolytic drugs (to dissolve the clot) can be very effective. If drug treatment fails transluminal angioplasty is often indicated--the narrowed part of the artery, clogged with atherosclerotic plaque or other deposits, can be stretched apart by passing a balloon to the site and gently inflating it a certain degree. In the event drug therapy is ineffective or angioplasty is too risky (often introduction of a balloon in an occluded artery can cause portions of the atherosclerotic material to become dislodged which may cause a total blockage at a point downstream of the subject occlusion thereby requiring emergency procedures), the procedure known as coronary artery bypass grafting (CABG) may be indicated. CABG is the most common and successful major heart operation performed, in America alone over 500,000 procedures being performed annually. The procedure takes at least two surgeons and can last up to five hours. First , the surgeon makes an incision down the center of the patient's chest and the heart is exposed by opening the pericardium. A length of vein is removed from another part of the body, typically the leg. The patient is connected to a heart-lung machine which takes over the function of the heart and lungs during the operation. The section of vein is first sewn to the aorta and then sewn onto a coronary artery at a place such that oxygenated blood can flow directly into the heart. The patient is then closed. Not only does the procedure require the installation of the heart-lung machine, a very risky procedure, but the sternum must be sawed through and the risk of infection is enhanced during the time the chest cavity is spread open.
Another method of improving myocardial blood supply is called transmyocardial revascularization (TMR), the creation of channels from the epicardial to the endocardial portions of the heart. The procedure using needles in a form of "myocardial acupuncture" has been experimented with at least as early as the 1930s and used clinically since the 1960s. Deckelbaum. L. I., Cardiovascular Applications of Laser technology, Lasers in Surgery and Medicine 15:315-341 (1994). The technique was said to relieve ischemia by allowing blood to pass from the ventricle through the channels either directly into other vessels perforated by the channels or into myocardial sinusoids which connect to the myocardial microcirculation. The procedure has been likened to transforming the human heart into one resembling that of a reptile.
In the reptile heart, perfusion occurs via communicating channels between the left ventricle and the coronary arteries. Frazier, O. H., Myocardial Revascularization with Laser--Preliminary Findings, Circulation, 1995; 92 [suppl II]:II-58-II-65. There is evidence of these communicating channels in the developing human embryo. In the human heart, myocardial microanatomy involves the presence of myocardial sinusoids. These sinusoidal communications vary in size and structure, but represent a network of direct arterial-luminal, arterial-arterial, arterial-venous, and venous-luminal connections. This vascular mesh forms an important source of myocardial blood supply in reptiles but its role in humans is poorly understood.
Numerous studies have been performed on TMR using lasers to bore holes in the myocardium. The exact mechanism by which blood flows into the myocardium is not well understood however. In one study, 20-30 channels per square centimeter were bored into the left ventricular myocardium of dogs prior to occlusion of the arteries. LAD ligation was conducted on both the revascularized animals as well as a set of control animals. Results showed that animals having undergone TMR prior to LAD ligation acutely showed no evidence of ischemia or infarction in contrast to the control animals. After sacrifice of the animals at ages between 4 weeks and 5 months, the laser-created channels could be demonstrated grossly and microscopically to be open and free of debris and scarring.
It is believed that the TMR channels occlude toward the epicardial surface but that their subendocardial section remains patent (unobstructed) and establishes camerosinusoidal connections. It is possible that the creation of laser channels in the myocardium may promote long-term changes that could augment myocardial blood flow such as by inducing angiogenesis in the region of the lased (and thus damaged) myocardium. Support of this possibility is reported in histological evidence of probable new vessel formation adjacent to collagen occluded transmyocardial channels. In the case of myocardial acupuncture or boring, which mechanically displaces or removes tissue, acute thrombosis followed by organization and fibrosis of clots is the principal mechanism of channel closure. By contrast, histological evidence of patent, endothelium-lined tracts within the laser-created channels supports the assumption that the inside of the laser channels is or can become hemocompatible and that it resists occlusion caused by thrombo-activation and/or fibrosis. A thin zone of charring occurs on the periphery of the laser-created transmyocardial channels through the well-known thermal effects of optical radiation on cardiovascular tissue. This type of interface may inhibit the immediate activation of the intrinsic clotting mechanisms because of the inherent hemocompatibility of carbon. In addition, the precise cutting action that results from the high absorption and low scattering of laser energy (CO.sub.2, HO, etc.) may minimize structural damage to collateral tissue, thus limiting the tissue thromboplastin-mediated activation of the extrinsic coagulation.
U.S. Pat. No. 4,658,817 issued Apr. 21, 1987 to Hardy teaches a method and apparatus for TMR using a laser. A surgical CO.sub.2 laser includes a handpiece for directing a laser beam to a desired location. Mounted on the forward end of the handpiece is a hollow needle to be used in surgical applications where the needle perforated a portion of tissue to provide the laser beam direct access to distal tissue.
U.S. Pat. No. 5,125,926 issued Jun. 30, 1992 to Rudko et al. teaches a heart-synchronized pulsed laser system for TMR. The device and method comprises a device for sensing the contraction and expansion of a beating heart. As the heart beat is monitored, the device triggers a pulse of laser energy to be delivered to the heart during a predetermined portion of the heartbeat cycle. This heart-synchronized pulsed laser system is important where the type of laser, the energy and pulse rate are potentially damaging to the beating heart or it's action. Often, application of laser energy to a beating heart can induce fibrillation or arrhythmia. Additionally, as the heart beats, it's spatial relationship between the heart and the tip of the laser delivery probe may change so that the necessary power of the beam and the required position of the handpiece may be unpredictable.
Finally, U.S. Pat. Nos. 5,380,316 issued Jan. 10, 1995 and 5,389,096 issued Feb. 14, 1995 both to Aita et al. teach systems and methods for intra-operative and percutaneous myocardial revascularization, respectively. The former patent is related to TMR performed by inserting a portion of an elongated flexible lasing apparatus into the chest cavity of a patient and lasing channels directly through the outer surface of the epicardium into the myocardium tissue. In the latter, TMR is performed by guiding an elongated flexible lasing apparatus into a patient's vasculature such that the firing end of the apparatus is adjacent the endocardium and lasing channels directly through the endocardium into the myocardium tissue without perforating the pericardium layer. These patents do not teach any method for controlling the elongated flexible laser delivery apparatus, nor do they teach methods of visualizing the areas of the heart being lased nor do they teach any method or devices for achieving TMR on surfaces or portions of the heart which are not directly accessible via a sternotomy, mini-sternotomy or via a trocar.
TMR is most often used to treat the lower left chamber of the heart. The lower chambers or ventricles are serviced by the more distal branches of the coronary arteries. Distal coronary arteries are more prone to blockage and resulting heart muscle damage. Roughly 50% of the left ventricle is direct line accessible through a thoracotomy or small incision between the ribs. However, roughly 50% is not direct line accessible and requires either rotating the heart or sliding around to the back side of the heart. Access to the heart is achieved by (1) sliding a device between the heart and pericardial sack which encases the heart, the device likely to have a 45-90 degree bend near the tip, (2) lifting the still beating heart, and (3) penetrating through the direct access side of the heart and/or through the septum of the heart. Lifting the still beating heart is less than desirable especially in patients with lowered heart performance. Furthermore, such manipulation can cause tachycardia (rapid beating of the heart absent undue exertion) fibrillation, arrhythmia or other interruptions in the normal beating cycle.
Thus, broadly, it is an object of the present invention to provide an improved method and device for laser-assisted TMR.
It is a further object of the present invention to provide an improved method and device for laser-assisted TMR in which the procedure may be carried out from within the interior of the heart and in which visualization or positioning of the laser delivery means is done by improved means.
It is a further object of the present invention to provide an improved method and device for laser-assisted TMR in which the procedure may be carried out on rear surfaces and other visually hidden external surfaces of the heart and in which visualization or positioning of the laser delivery means is done by improved means.
It is a further object of the present invention to provide an improved method and device for laser-assisted TMR in which the need for visualization during the procedure is minimized by employing a depth stop, such as a mesh basket or "moly bolt" device adjacent the end of the elongated flexible laser delivery means, to position the distal end of the laser delivery means
It is a further object of the present invention to provide an improved method and device for laser-assisted TMR in which the distal end of the elongated flexible laser delivery means comprises a known visualization system with an adjunct visualization device to enhance the quality of the visualized image.
It is a further object of the present invention to provide an improved method and device for laser-assisted TMR in which the need for visualization during the procedure is minimized by deploying a mechanical or other tether coupled to the heart at a point adjacent to the area to be revascularized, thereby conveying the distal end of the laser delivery means to that area and maintaining it in a predetermined position during the creation of channels within the tissue of the myocardium.